Method and system for add-on battery

ABSTRACT

Systems and methods for operating an add-on battery that may be electrically coupled to a second system that includes an electrical energy storage device are presented. In one example, the systems and methods provide for extending operation of the second system via selectively powering the second system via the add-on battery in response to operating conditions of the second system.

PRIORITY

The present application is related to, claims the priority benefit of,and is a U.S. continuation patent application of, U.S. patentapplication Ser. No. 16/808,261, filed on Mar. 3, 2020 and issued asU.S. Pat. No. 11,133,694, which is related to, claims the prioritybenefit of, and is a U.S. continuation patent application of, U.S.patent application Ser. No. 15/792,192, filed Oct. 24, 2017 and issuedas U.S. Pat. No. 10,581,261 on Mar. 3, 2020, which is related to, claimsthe priority benefit of, and is a U.S. continuation patent applicationof, U.S. patent application Ser. No. 14/630,535, filed Feb. 24, 2015 andissued as U.S. Pat. No. 9,800,071 on Oct. 24, 2017, the contents ofwhich are incorporated into the present disclosure directly and byreference in their entirety.

FIELD

The present description relates to methods and a system for an add-onbattery that may be electrically coupled to an external battery of asystem that relies at least partially on power from the externalbattery. The methods and systems may be particularly useful for systemsthat operate remotely or untethered from a stationary power grid.

BACKGROUND SUMMARY

A system may include electrically operated devices for performing tasksand/or providing enjoyment to a user. For example, a system maycomprise, but is not limited to including motors, solenoids, radioreceivers, display devices, computers, and cooking appliances.Electrically operated devices in the system may be powered via anelectrical energy storage device such as a battery or capacitor when thesystem is remote or untethered from a stationary power grid. Theelectrical energy storage device may be adequate for operating theelectrically operated devices for a limited amount of time or chargeconsumption, but the electrical energy storage devices have limitedcapacity. Consequently, the electrical energy storage device may beperiodically returned to the stationary power grid for recharging.Further, the system may be designed to operate with lower charge densityenergy storage devices (e.g., lead-acid batteries), thereby, potentiallyincreasing the frequency of system charging. Therefore, it may bedesirable to increase operating time of a system that is electricallypowered via an electrical energy storage device so that less frequentenergy storage device charging may be necessary.

The inventors herein have recognized the above-mentioned issue and havedeveloped an add-on battery system, comprising: a battery; abi-directional DC/DC converter; and a controller including instructionsstored in non-transitory or non-volatile memory to direct current flowinto the battery and out of the battery via the bi-directional DC/DCconverter in response to conditions of an external electrical energystorage device.

By controlling current flow into and out of a battery of an add-onbattery system in response to conditions of an external electricalenergy storage device via a bi-directional DC/DC converter, it may bepossible to extend operating time of the external electrical energystorage device and the external system in which it operates. Further,electrical charge stored in the add-on battery system may be applied tothe external system more efficiently. For example, charge from theadd-on battery system may only be delivered during some conditions inresponse to an electrical load being applied to the external electricalenergy storage device. In particular, the add-on battery system may onlysupply electrical power (e.g., current and voltage) when the externalelectrical energy storage device has a state of charge greater than athreshold and when the external load is consuming power from theexternal electrical energy storage device. Consequently, the add-onbattery system supplies electrical power during conditions when it maybe most efficient. Use of add-on battery power may be more efficientwhen a load is being supplied to the external electrical energy storagedevice because more electrical power may be applied to the electricalload rather than to less efficient charging of the external electricalenergy storage device. Additionally, the bi-directional DC/DC converterallows the add-on battery system to be charged via a charger thatcharges the external system battery.

The present description may provide several advantages. In particular,the approach may improve efficiency of electrical power transfer fromone electrical system to another. In addition, the approach may alsoprovide energy transfer between systems without having to provide largecables between electrical systems. Further, the add-on battery systemmay be electrically coupled to an external system at any time withouthaving to deactivate the external system. Further still, the add-onbattery system may interfere less with fuel gauge displays of theexternal system.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment, referred to herein as the DetailedDescription, when taken alone or with reference to the drawings, where:

FIG. 1 is a schematic diagram of an add-on battery system;

FIGS. 2 and 3 are schematic diagrams of example systems that may benefitfrom the add-on battery system of FIG. 1;

FIG. 4 is a high level example electrical schematic of an example add-onbattery system;

FIGS. 5-8 are plots showing signals of interest in different operationalmodes of a bi-directional DC\DC converter included in the example add-onbattery system; and

FIGS. 9-15 show an example method for operating the add-on batterysystem.

DETAILED DESCRIPTION

The present description is related to augmenting electrical powersupplied to a system that operates using electrical power supplied viaan electrical energy storage device. Electrical power may be provided tothe system via an add-on battery system as is shown in FIG. 1. Theadd-on battery system of FIG. 1 may be applied to systems that operateusing electrical power supplied via an electrical energy storage devicesuch as the systems shown in FIGS. 2 and 3. The add-on battery systemmay include an electrical configuration as is shown in FIG. 4. Theadd-on battery system may include a bi-directional DC/DC converter thatoperates as is shown in FIGS. 5-8. The add-on battery system may beoperated according to the method of FIGS. 9-15.

Referring now to FIG. 1, a plan view of add-on battery system 100 isshown. Add-on battery system 100 is shown including a case 102 andelectrical connector 104. In some examples, case 102 may include a carryhandle (not shown) for transporting the add-on battery to a remotelocation where stationary grid power is out of range or to systems thatchange location frequently. Electrical connector 104 may provideelectrical coupling between add-on battery system 100 and an externalsystem that is powered via an electrical energy storage device (notshown). In some examples, add-on battery system 100 may include aplurality of electrical connectors. An external system in context ofthis disclosure refers to a system that is not part of the add-onbattery system. An external electrical energy storage device refers toan electrical energy storage device that is not part of the add-onbattery system.

Referring now to FIG. 2, a schematic view of a powered chair 200 isshown. Powered chair 200 may propel an occupant (not shown) to a desiredlocation. Powered chair 200 includes a battery 202 for supplyingelectrical power to a motor that is mechanically coupled to wheels 208.Add-on battery system 100 is shown electrically coupled to battery 202via cable 204. Add-on battery system 100 supplies electrical power tothe motor (not shown) when the motor is activated. In some examples,add-on battery system 100 does not charge battery 202 when the motor isnot activated unless charge of battery 202 is determined to be less thana threshold level. By supplying a greater percentage of charge fromadd-on battery system 100 to the motor (not shown) rather than battery202, charge supplied by add-on battery system 100 may be used moreefficiently, thereby conserving charge of battery 202. Thus, anoperating range of powered chair 200 may be increased via electricallycoupling add-on battery system 100 to battery 202.

Referring now to FIG. 3, a schematic view of a vehicle 300 is shown.Vehicle 300 may deliver an occupant (not shown) to a desired location.Vehicle 300 includes a battery 302 for supplying electrical power to amotor that is mechanically coupled to wheels 308. Alternatively, battery302 may supply electrical power to a starter motor (not shown) forrotating an engine (not shown). Add-on battery system 100 is shownelectrically coupled to battery 302 via cable 304. Add-on battery system100 supplies electrical power to the motor (not shown) or starter (notshown) when the motor or starter is activated. Alternatively, if chargeof battery 302 is less than a threshold, add-on battery system 100 maysupply charge to battery 302 so that battery 302 may supply a higherlevel of current to the starter motor (not shown) at a later time afterbattery 302 is charged. Add-on battery system 100 does not chargebattery 302 when the motor or starter motor is not activated unlesscharge of battery 302 is determined to be less than a threshold level.By supplying a greater percentage of charge from add-on battery system100 to the motor (not shown) rather than battery 302, charge supplied byadd-on battery system 100 may be used more efficiently and charge ofbattery 302 may be conserved. However, when charge of battery 302 islow, add-on battery system 100 may charge battery 302 to ensure battery302 may supply higher current levels to the motor or starter motor thanthat which may be provided solely via add-on battery system 100. Thus,add-on battery system 100 may improve the possibility of engine startingvia a starter motor. Further, add-on battery system 100 may increase theoperating range of vehicle 300 when vehicle 300 is propelled via a motorby electrically coupling add-on battery system 100 to battery 302.

It should be noted that powered chair 200 and vehicle 300 are only twoexample systems that may be assisted via add-on battery system 100. Thescope of systems that add-on battery system 100 may assist is notlimited to powered chair 200 and vehicle 300. For example, add-onbattery system may be applied to battery powered cooking appliances, alltypes of battery powered vehicles including manned and unmanned batterypowered vehicles, radios, displays, computers, motors, solenoids, andany other type of battery powered device.

Referring now to FIG. 4, a high level electrical schematic of an exampleadd-on battery electrical system is shown. The electrical system 400 ofFIG. 4 may be incorporated into the add-on battery system 100 shown inFIG. 1. In other words, the electrical system 400 of FIG. 4 may be asub-system of the add-on battery system 100 of FIG. 1. Further, theelectrical system 400 of FIG. 4 may provide the operating sequences ofFIGS. 5-8, and the electrical system 400 of FIG. 4 may include themethod of FIGS. 9-15 stored in non-transitory memory. Additionally, FIG.4 is only one example of an electrical system described by Applicant'sclaims. Further systems and methods derived from or made obvious fromApplicant's specification and provided by one skilled in the art arealso within the scope of this specification.

Add-on battery system 100 includes electrical system 400. Components ofelectrical system 400 are shown within the dotted line indicating thebounds of add-on battery system 100. Electrical system 400 includes abattery cell management system (BMS) 402. BMS 402 includes electronicsfor sensing and/or inferring voltage, temperature, battery current, andbattery state of charge of battery 405. If charge of one or more batterycells 406 is greater than desired, BMS 402 may discharge one or more ofbattery cells 406 into variable resistor 404, thereby, balancing chargewithin battery 405. BMS 402 may provide an indication to add-on batterysystem controller 416 that battery 405 is in condition for dischargingvia conductor 430. BMS 402 may also provide an indication to add-onbattery system controller 416 that battery 405 is in condition forcharging via conductor 431. Battery 405 may be comprised of one or morebattery cells 406. Battery cells 406 may be arranged in series andparallel to increase battery voltage and charge capacity. Battery 405may be charged or discharged via H-bridge 475. Capacitor 410 filtersoutput of battery 405.

Add-on battery system controller 416 includes a central processing unitor digital signal processing unit 470, random-access memory 471,read-only or non-transitory memory 472, and inputs/outputs 473. Add-onbattery system controller 416 receives charge and discharge current fromoperational amplifiers 430 and 432. Add-on battery system controller 416sends H-bridge control signals to metal oxide semiconductor field effecttransistor (MOSFET) driver 412. Further, add-on battery systemcontroller 416 may receive battery current and battery voltageinformation from external battery operated system 485 via a controllerarea network (CAN) 496 and/or isolated analog channels 482. Centralprocessing unit or digital signal processing unit 470 may executeinstructions representing the method of FIGS. 9-15. The executableinstructions of FIGS. 9-15 may be stored in non-transitory memory 472.

MOSFET driver 412 includes devices for sourcing current and voltagesufficient for driving MOSFETS 418-421. MOSFET driver 412 receivesH-bridge control signals from add-on battery system controller 416.H-bridge 475 is comprised of MOSFETS 418-421, transformer 440, andinductor 442 arranged in vertical and horizontal circuits forming a Hshape. Each of MOSFETS 418-421 are shown as N-channel MOSFETS, butP-channel MOSFETS or other switching devices may be substituted forMOSFETS 418-421 in other examples. MOSFETS 418-421 include drains 461,gates 460, and sources 462. Shottky diodes 465 are shown in parallelwith MOSFETS 418421. A first vertical leg circuit 454 of H-bridge 475 isformed by electrically coupling source 462 of MOSFET 418 to drain ofMOSFET 419. A second vertical leg circuit 455 of H-bridge 475 is formedby electrically coupling source 462 of MOSFET 420 to drain of MOSFET421. Sources of MOSFET 419 and MOSFET 421 are electrically coupled tothe low potential side of battery 405. Drains of MOSFET 418 and MOSFET420 are electrically coupled to the high potential side of battery 405.The horizontal leg circuit 456 electrically couples first 454 and second455 vertical legs. Horizontal leg circuit 456 includes transformer 440and inductor 442. Transformer 440 is electrically coupled to operationalamplifiers 430 and 432, thereby providing feedback of inductor currentto add-on battery system controller 416. Resistor 436 electricallycouples inverting inputs of operational amplifiers 430 and 432. H-bridgeoutput or input (e.g., current and voltage), input/output depending oncontrol of H-bridge MOSFETS, is applied to electrical connector 104 viaconductors 498 and 499. MOSFET 424 may be selectively activated tofilter H-bridge output via capacitor 444. MOSFET 418 may be referred toas transistor Q1. MOSFET 419 may be referred to as transistor Q2. MOSFET420 may be referred to as transistor Q3. MOSFET 421 may be referred toas transistor Q4. The low potential side of battery 405 is electricallycoupled to ground 414 via resistor 408.

Add-on battery system 100 is shown in electrical communication withexternal system 485 via electrical connector 104. External system 485may include an external electrical energy storage device (e.g., abattery) 490 and an electrical load 492. Electrical load 492 may becomprised of a motor, display, electrical actuator, or other electricalload. However, in some examples, electrical load 492 may operate in somemodes as a generator supplying charge to external energy storage device490. In some examples, current sensor 486 may indicate current flowbetween external electrical energy storage device 490 and electricalload 492 via conductor 488. Current information from electrical energystorage device 490 to electrical load 492 may delivered to add-onbattery system 100 via CAN 496. Controller 494 may process current datafrom sensor 486 and output the data via CAN 496. Conductors 480 mayelectrically couple H-bridge 475 and electrical energy storage device490. Voltage of external electrical energy storage device 490 may besupplied to add-on battery system controller 416 via conductors 482.H-bridge 475 may operate supplying current to external electrical energystorage device 490 from battery 405. Alternatively, H-bridge 475 mayoperate supplying current to battery 405 from external electrical energystorage device 490. Thus, H-bridge 475 provides bi-directional powerflow. Further, H-bridge 475 may be operated in buck or boost modesdepending on the voltage difference and the direction of current flowbetween external electrical energy storage device 490 and battery 405.

Thus, the system of FIGS. 1-4 provides for an add-on battery system,comprising: a battery; a bi-directional DC/DC converter; and acontroller including instructions stored in non-transitory memory todirect current flow into the battery and out of the battery via thebi-directional DC/DC converter in response to conditions of an externalelectrical energy storage device. The add-on battery system furthercomprises additional instructions to activate the bi-directional DC/DCconverter in response to a derivative of an external electrical energystorage device voltage. The add-on battery system further comprisesadditional instructions to activate the bi-directional DC/DC converterin response to an error between a rolling average of an externalelectrical energy storage device voltage and an instantaneous externalelectrical energy storage device voltage.

In some examples, the add-on battery system includes where thebi-directional DC/DC converter includes an H-bridge comprising atransformer positioned in a horizontal circuit extending between twovertical circuits. The add-on battery system includes where the twovertical circuits are comprised of metal oxide semiconductor fieldeffect transistors. The add-on battery system includes where thehorizontal circuit further comprises an inductor. The add-on batterysystem further comprises additional instructions to deactivate thebi-directional DC/DC converter in response to a voltage of an externalelectrical energy storage device being less than a first threshold andbeing greater than a second threshold. The add-on battery system furthercomprises additional instructions to activate the bi-directional DC/DCconverter in response to the voltage of the external electrical energystorage device being greater than the first threshold and being lessthan the second threshold.

Referring now to FIG. 5, an operating sequence for a bi-directionalH-bridge supplying charge from an add-on battery system battery to anexternal electrical energy storage device is shown. The sequence of FIG.5 may be provided by the H-bridge of FIG. 4 according to the method ofFIGS. 9-15. Times of interest in the sequence are indicated via verticalmarkers T1-T3. The sequence of FIG. 5 applies when voltage of the add-onbattery system battery voltage is lower than voltage of the externalelectrical energy storage device. This mode may be referred to as aboost mode.

The first plot from the top of FIG. 5 is a plot of voltage applied to agate of MOSFET Q1 of FIG. 4 versus. MOSFET Q1 conducts and allowscurrent to flow between the drain and source of Q1 when the voltageapplied to the gate of MOSFET Q1 is a higher value. Voltage applied tothe gate of MOSFET Q1 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q1 is near ground potentialnear the horizontal axis. MOSFET Q1 does not conduct and current doesnot flow from the drain to source of MOSFET Q1 when the voltage appliedto the gate of MOSFET Q1 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The second plot from the top of FIG. 5 is a plot of voltage applied to agate of MOSFET Q2 of FIG. 4 versus time. MOSFET Q2 conducts and allowscurrent to flow between the drain and source of Q2 when the voltageapplied to the gate of MOSFET Q2 is a higher value. Voltage applied tothe gate of MOSFET Q2 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q2 is near ground potentialnear the horizontal axis. MOSFET Q2 does not conduct and current doesnot flow from the drain to source of MOSFET Q2 when the voltage appliedto the gate of MOSFET Q2 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The third from the top of FIG. 5 is a plot of voltage applied to a gateof MOSFET Q4 of FIG. 4 versus time. MOSFET Q4 conducts and allowscurrent to flow between the drain and source of Q4 when the voltageapplied to the gate of MOSFET Q4 is a higher value. Voltage applied tothe gate of MOSFET Q4 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q4 is near ground potentialnear the horizontal axis. MOSFET Q4 does not conduct and current doesnot flow from the drain to source of MOSFET Q4 when the voltage appliedto the gate of MOSFET Q4 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fourth plot from the top of FIG. 5 is a plot of voltage applied to agate of MOSFET Q3 of FIG. 4 versus time. MOSFET Q3 conducts and allowscurrent to flow between the drain and source of Q3 when the voltageapplied to the gate of MOSFET Q3 is a higher value. Voltage applied tothe gate of MOSFET Q3 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q3 is near ground potentialnear the horizontal axis. MOSFET Q3 does not conduct and current doesnot flow from the drain to source of MOSFET Q3 when the voltage appliedto the gate of MOSFET Q3 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fifth plot from the top of FIG. 5 is a plot of H-bridge inductorcurrent (e.g., 442 of FIG. 4) versus time. The H-bridge inductor currentis based on the operating states of MOSFETS Q1-Q4. Inductor currentincreases in the direction of the vertical axis arrow. Inductor currentis near zero at the horizontal axis. The horizontal axis represents timeand time increases from the left side of the plot to the right side ofthe plot.

At time T1, voltage applied to the gate of Q1 is a high level so Q1conducts. The voltage applied to the gate of Q2 is a low level so Q2does not conduct. The voltage applied to Q4 transitions from a low levelto a high level so that Q4 begins to conduct. The voltage applied to Q3is at a low level so Q3 is not conducting at time T1. Inductor currentLI begins to increase when Q4 begins to conduct because current flowsfrom the add-on battery system battery to the H-bridge inductor. Currentmay flow into the inductor for a predetermined amount of time or until aspecified current flow into the inductor is achieved. The amount ofcurrent flowing into the inductor determines the amount of energy storedin the inductor.

At time T2, voltage applied to the gate of Q1 remains at a high level soQ1 conducts. The voltage applied to the gate of Q2 also remains at a lowlevel so Q2 does not conduct. The voltage applied to Q4 transitions fromthe higher level to the lower level so that Q4 stops conducting. Thevoltage applied to Q3 is at a low level so Q3 is not conducting at timeT2, but it transitions to a higher level shortly thereafter so that Q3begins to conduct. Inductor current LI begins to decrease in response toQ4 being turned off and Q3 being turned on. Energy stored in a field ofthe inductor is released to the external electrical energy storagedevice when Q3 begins to conduct. The voltage on the side of theinductor closest to the external electrical energy storage device isincreased to a level greater than that of the add-on battery system'sbattery because, referenced to ground, the inductor voltage includes theadd-on battery system's battery voltage plus the inductor's voltagewhich may be expressed as:

${V = {L \cdot \frac{di}{dt}}},$

where V is voltage across the injector, L is the inductor's inductance,i is current, and t is time. Thus, Q3 is activated shortly after Q4 isdeactivated so that the transistors are operated sequentially (e.g., oneafter the other).

Just before time T3, voltage applied to the gate of Q1 is a high levelso Q1 conducts. The voltage applied to the gate of Q2 is a low level soQ2 does not conduct. The voltage applied to Q4 is at a low level so Q4does not conduct. The voltage applied to Q3 transitions to a low levelso Q3 stops conducting and the inductor stops discharging. At time T3,Q4 is reactivated by applying a higher voltage to the gate of Q4 and thesequence resumes similar as the sequence began at time T1.

In this way, charge stored in the add-on battery system may betransferred to the external electrical energy storage device. Further,charge may be transferred from the add-on battery system even thoughbattery voltage of the add-on system is lower than battery voltage ofthe external electrical energy storage device.

Referring now to FIG. 6, an operating sequence for a bi-directionalH-bridge supplying charge from an external electrical energy storagedevice to an add-on battery system battery is shown. The sequence ofFIG. 6 may be provided by the H-bridge of FIG. 4 according to the methodof FIGS. 9-15. Times of interest in the sequence are indicated viavertical markers T4-T6. The sequence of FIG. 6 applies when voltage ofthe add-on battery system battery voltage is higher than voltage of theexternal electrical energy storage device. This mode may be referred toas a boost mode.

The first plot from the top of FIG. 6 is a plot of voltage applied to agate of MOSFET Q3 of FIG. 4 versus. MOSFET Q3 conducts and allowscurrent to flow between the drain and source of Q3 when the voltageapplied to the gate of MOSFET Q3 is a higher value. Voltage applied tothe gate of MOSFET Q3 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q3 is near ground potentialnear the horizontal axis. MOSFET Q3 does not conduct and current doesnot flow from the drain to source of MOSFET Q3 when the voltage appliedto the gate of MOSFET Q3 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The second plot from the top of FIG. 6 is a plot of voltage applied to agate of MOSFET Q4 of FIG. 4 versus time. MOSFET Q4 conducts and allowscurrent to flow between the drain and source of Q4 when the voltageapplied to the gate of MOSFET Q4 is a higher value. Voltage applied tothe gate of MOSFET Q4 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q4 is near ground potentialnear the horizontal axis. MOSFET Q4 does not conduct and current doesnot flow from the drain to source of MOSFET Q4 when the voltage appliedto the gate of MOSFET Q4 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The third from the top of FIG. 6 is a plot of voltage applied to a gateof MOSFET Q2 of FIG. 4 versus time. MOSFET Q2 conducts and allowscurrent to flow between the drain and source of Q2 when the voltageapplied to the gate of MOSFET Q2 is a higher value. Voltage applied tothe gate of MOSFET Q2 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q2 is near ground potentialnear the horizontal axis. MOSFET Q2 does not conduct and current doesnot flow from the drain to source of MOSFET Q2 when the voltage appliedto the gate of MOSFET Q2 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fourth plot from the top of FIG. 6 is a plot of voltage applied to agate of MOSFET Q1 of FIG. 4 versus time. MOSFET Q1 conducts and allowscurrent to flow between the drain and source of Q1 when the voltageapplied to the gate of MOSFET Q1 is a higher value. Voltage applied tothe gate of MOSFET Q1 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q1 is near ground potentialnear the horizontal axis. MOSFET Q1 does not conduct and current doesnot flow from the drain to source of MOSFET Q1 when the voltage appliedto the gate of MOSFET Q1 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fifth plot from the top of FIG. 6 is a plot of K-bridge inductorcurrent (e.g., 442 of FIG. 4) versus time. The H-bridge inductor currentis based on the operating states of MOSFETS Q1-Q4. Inductor currentincreases in the direction of the vertical axis arrow. Inductor currentis near zero at the horizontal axis. The horizontal axis represents timeand time increases from the left side of the plot to the right side ofthe plot.

At time T4, voltage applied to the gate of Q3 is a high level so Q3conducts. The voltage applied to the gate of Q4 is a low level so Q4does not conduct. The voltage applied to Q2 transitions from a low levelto a high level so that Q2 begins to conduct. The voltage applied to Q1is at a low level so Q1 is not conducting at time T1. Inductor currentLI begins to increase when Q2 begins to conduct because current flowsfrom the external electrical energy storage device to the K-bridgeinductor. Current may flow into the inductor for a predetermined amountof time or until a specified current flow into the inductor is achieved.The amount of current flowing into the inductor determines the amount ofenergy stored in the inductor.

At time T5, voltage applied to the gate of Q3 remains at a high level soQ3 conducts. The voltage applied to the gate of Q4 also remains at a lowlevel so Q4 does not conduct. The voltage applied to Q2 transitions fromthe higher level to the lower level so that Q2 stops conducting. Thevoltage applied to Q1 is at a low level so Q1 is not conducting at timeT5, but it transitions to a higher level shortly thereafter so that Q1begins to conduct. Inductor current LI begins to decrease in response toQ2 being turned off and Q1 being turned on. Energy stored in a field ofthe inductor is released to the add-on battery system battery when Q1begins to conduct. The voltage on the side of the inductor closest tothe add-on battery system battery is increased to a level greater thanthat of the external electrical energy storage device because theinductor voltage includes the external electrical energy storagedevice's voltage plus the inductor's voltage. Thus, Q1 is activatedshortly after Q2 is deactivated so that the transistors are operatedsequentially (e.g., one after the other).

Just before time T6, voltage applied to the gate of Q3 is a high levelso Q3 conducts. The voltage applied to the gate of Q4 is a low level soQ4 does not conduct. The voltage applied to Q2 is at a low level so Q2does not conduct. The voltage applied to Q1 transitions to a low levelso Q1 stops conducting and the inductor stops discharging. At time T6,Q2 is reactivated by applying a higher voltage to the gate of Q2 and thesequence resumes similar as the sequence began at time T4.

In this way, charge stored in the external electrical energy storagedevice may be transferred to the add-on battery system. Further, chargemay be transferred from the external electrical energy storage deviceeven though battery voltage of the external electrical energy storagedevice is lower than battery voltage of the add-on battery system.

Referring now to FIG. 7, an operating sequence for a bi-directionalH-bridge supplying charge from an add-on battery system battery to anexternal electrical energy storage device is shown. The sequence of FIG.7 may be provided by the H-bridge of FIG. 4 according to the method ofFIGS. 9-15. Times of interest in the sequence are indicated via verticalmarkers T7-T9. The sequence of FIG. 7 applies when voltage of the add-onbattery system battery voltage is higher than voltage of the externalelectrical energy storage device. This mode may be referred to as a buckmode.

The first plot from the top of FIG. 7 is a plot of voltage applied to agate of MOSFET Q3 of FIG. 4 versus. MOSFET Q3 conducts and allowscurrent to flow between the drain and source of Q3 when the voltageapplied to the gate of MOSFET Q3 is a higher value. Voltage applied tothe gate of MOSFET Q3 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q3 is near ground potentialnear the horizontal axis. MOSFET Q3 does not conduct and current doesnot flow from the drain to source of MOSFET Q3 when the voltage appliedto the gate of MOSFET Q3 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The second plot from the top of FIG. 7 is a plot of voltage applied to agate of MOSFET Q4 of FIG. 4 versus time. MOSFET Q4 conducts and allowscurrent to flow between the drain and source of Q4 when the voltageapplied to the gate of MOSFET Q4 is a higher value. Voltage applied tothe gate of MOSFET Q4 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q4 is near ground potentialnear the horizontal axis. MOSFET Q4 does not conduct and current doesnot flow from the drain to source of MOSFET Q4 when the voltage appliedto the gate of MOSFET Q4 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The third from the top of FIG. 7 is a plot of voltage applied to a gateof MOSFET Q1 of FIG. 4 versus time. MOSFET Q1 conducts and allowscurrent to flow between the drain and source of Q1 when the voltageapplied to the gate of MOSFET Q1 is a higher value. Voltage applied tothe gate of MOSFET Q1 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q1 is near ground potentialnear the horizontal axis. MOSFET Q1 does not conduct and current doesnot flow from the drain to source of MOSFET Q1 when the voltage appliedto the gate of MOSFET Q1 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fourth plot from the top of FIG. 7 is a plot of voltage applied to agate of MOSFET Q2 of FIG. 4 versus time. MOSFET Q2 conducts and allowscurrent to flow between the drain and source of Q2 when the voltageapplied to the gate of MOSFET Q2 is a higher value. Voltage applied tothe gate of MOSFET Q2 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q2 is near ground potentialnear the horizontal axis. MOSFET Q2 does not conduct and current doesnot flow from the drain to source of MOSFET Q2 when the voltage appliedto the gate of MOSFET Q2 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fifth plot from the top of FIG. 7 is a plot of H-bridge inductorcurrent (e.g., 442 of FIG. 4) versus time. The H-bridge inductor currentis based on the operating states of MOSFETS Q1-Q4. Inductor currentincreases in the direction of the vertical axis arrow. Inductor currentis near zero at the horizontal axis. The horizontal axis represents timeand time increases from the left side of the plot to the right side ofthe plot.

At time T7, voltage applied to the gate of Q3 is a high level so Q3conducts. The voltage applied to the gate of Q4 is a low level so Q4does not conduct. The voltage applied to Q1 transitions from a low levelto a high level so that Q1 begins to conduct. The voltage applied to Q2is at a low level so Q2 is not conducting at time T7. Inductor currentLI begins to increase when Q1 begins to conduct because current flowsfrom the add-on battery system battery to the H-bridge inductor. Theincreasing inductor current induces an opposing voltage in the inductor,the opposing voltage being less than the source voltage (e.g., theadd-on battery system battery voltage). Current may flow into theinductor for a predetermined amount of time or until a specified currentflow into the inductor is achieved. The amount of current flowing intothe inductor determines the amount of energy stored in the inductor.

At time T8, voltage applied to the gate of Q3 remains at a high level soQ3 conducts. The voltage applied to the gate of Q4 remains at a lowlevel so Q4 does not conduct. The voltage applied to Q1 transitions fromthe higher level to the lower level so that Q1 stops conducting, but theinductor remains electrically coupled to the external electrical energystorage device instead of remaining coupled to the add-on battery systembattery as in the case of boost mode. The voltage applied to Q2 is at alow level so Q2 is not conducting at time T8, but it transitions to ahigher level shortly thereafter so that Q2 begins to conduct. Inductorcurrent LI begins to decrease in response to Q1 being turned off and Q2being turned on. Energy stored in a field of the inductor is released tothe external electrical energy storage device when Q2 begins to conduct.Thus, Q2 is activated shortly after Q1 is deactivated so that thetransistors are operated sequentially (e.g., one after the other).

Just before time T9, voltage applied to the gate of Q3 is a high levelso Q3 conducts. The voltage applied to the gate of Q4 is a low level soQ4 does not conduct. The voltage applied to Q1 is at a low level so Q1does not conduct. The voltage applied to Q2 transitions to a low levelso Q2 stops conducting and the inductor stops discharging. At time T9,Q1 is reactivated by applying a higher voltage to the gate of Q1 and thesequence resumes similar as the sequence began at time T7.

In this way, charge stored in the add-on battery system may betransferred to the external electrical energy storage device. Further,charge may be transferred from the add-on battery system even thoughbattery voltage of the add-on system is higher than battery voltage ofthe external electrical energy storage device.

Referring now to FIG. 8, an operating sequence for a bi-directionalH-bridge supplying charge from an external electrical energy storagedevice to an add-on battery system battery is shown. The sequence ofFIG. 8 may be provided by the H-bridge of FIG. 4 according to the methodof FIGS. 9-15. Times of interest in the sequence are indicated viavertical markers T10-T12. The sequence of FIG. 8 applies when voltage ofthe add-on battery system battery voltage is less than voltage of theexternal electrical energy storage device. This mode may be referred toas a buck mode.

The first plot from the top of FIG. 8 is a plot of voltage applied to agate of MOSFET Q1 of FIG. 4 versus. MOSFET Q1 conducts and allowscurrent to flow between the drain and source of Q1 when the voltageapplied to the gate of MOSFET Q1 is a higher value. Voltage applied tothe gate of MOSFET Q1 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q1 is near ground potentialnear the horizontal axis. MOSFET Q1 does not conduct and current doesnot flow from the drain to source of MOSFET Q1 when the voltage appliedto the gate of MOSFET Q1 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The second plot from the top of FIG. 8 is a plot of voltage applied to agate of MOSFET Q2 of FIG. 4 versus time. MOSFET Q2 conducts and allowscurrent to flow between the drain and source of Q2 when the voltageapplied to the gate of MOSFET Q2 is a higher value. Voltage applied tothe gate of MOSFET Q2 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q2 is near ground potentialnear the horizontal axis. MOSFET Q2 does not conduct and current doesnot flow from the drain to source of MOSFET Q2 when the voltage appliedto the gate of MOSFET Q2 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The third from the top of FIG. 8 is a plot of voltage applied to a gateof MOSFET Q3 of FIG. 4 versus time. MOSFET Q3 conducts and allowscurrent to flow between the drain and source of Q3 when the voltageapplied to the gate of MOSFET Q3 is a higher value. Voltage applied tothe gate of MOSFET Q3 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q3 is near ground potentialnear the horizontal axis. MOSFET Q3 does not conduct and current doesnot flow from the drain to source of MOSFET Q3 when the voltage appliedto the gate of MOSFET Q3 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fourth plot from the top of FIG. 8 is a plot of voltage applied to agate of MOSFET Q4 of FIG. 4 versus time. MOSFET Q4 conducts and allowscurrent to flow between the drain and source of Q4 when the voltageapplied to the gate of MOSFET Q4 is a higher value. Voltage applied tothe gate of MOSFET Q4 increases in the direction of the vertical axisarrow. Voltage applied to the gate of MOSFET Q4 is near ground potentialnear the horizontal axis. MOSFET Q4 does not conduct and current doesnot flow from the drain to source of MOSFET Q4 when the voltage appliedto the gate of MOSFET Q4 is near ground potential. The horizontal axisrepresents time and time increases from the left side of the plot to theright side of the plot.

The fifth plot from the top of FIG. 8 is a plot of H-bridge inductorcurrent (e.g., 442 of FIG. 4) versus time. The K-bridge inductor currentis based on the operating states of MOSFETS Q1-Q4. Inductor currentincreases in the direction of the vertical axis arrow. Inductor currentis near zero at the horizontal axis. The horizontal axis represents timeand time increases from the left side of the plot to the right side ofthe plot.

At time T10, voltage applied to the gate of Q1 is a high level so Q1conducts. The voltage applied to the gate of Q2 is a low level so Q2does not conduct. The voltage applied to Q3 transitions from a low levelto a high level so that Q2 begins to conduct. The voltage applied to Q4is at a low level so Q4 is not conducting at time T10. Inductor currentLI begins to increase when Q3 begins to conduct because current flowsfrom the external electrical energy storage device to the H-bridgeinductor. Current may flow into the inductor for a predetermined amountof time or until a specified current flow into the inductor is achieved.The amount of current flowing into the inductor determines the amount ofenergy stored in the inductor.

At time T11, voltage applied to the gate of Q1 remains at a high levelso Q1 conducts. The voltage applied to the gate of Q2 remains at a lowlevel so Q2 does not conduct. The voltage applied to Q3 transitions fromthe higher level to the lower level so that Q3 stops conducting. Thevoltage applied to Q4 is at a low level so Q4 is not conducting at timeT1, but it transitions to a higher level shortly thereafter so that Q4begins to conduct. Inductor current LI begins to decrease in response toQ3 being turned off and Q4 being turned on. Energy stored in a field ofthe inductor is released to the add-on battery system battery when Q4begins to conduct. The voltage across the inductor is less than theexternal electrical energy storage device voltage. Thus, Q4 is activatedshortly after Q3 is deactivated so that the transistors are operatedsequentially (e.g., one after the other).

Just before time T12, voltage applied to the gate of Q1 is a high levelso Q1 conducts. The voltage applied to the gate of Q2 is a low level soQ2 does not conduct. The voltage applied to Q3 is at a low level so Q3does not conduct. The voltage applied to Q4 transitions to a low levelso Q4 stops conducting and the inductor stops discharging. At time T12,Q3 is reactivated by applying a higher voltage to the gate of Q3 and thesequence resumes similar as the sequence began at time T10.

In this way, charge stored in the external electrical energy storagedevice may be transferred to the add-on battery system. Further, chargemay be transferred from the external electrical energy storage deviceeven though battery voltage of the external electrical energy storagedevice is higher than battery voltage of the add-on battery system.

Thus, the H-bridge may be operated in a buck or boost mode to supplycharge to the external electrical energy storage device voltage from theadd-on battery system battery or to supply charge to the add-on batterysystem battery from the external electrical energy storage devicevoltage. By allowing charge to flow from the external electrical energystorage device voltage to the add-on battery system battery, the add-onbattery system battery may be recharged when the external electricalenergy storage device voltage is recharged by a battery charger. Thus,the add-on battery system battery does not require a dedicated chargerto be recharged.

Referring now to FIGS. 9-15, an example method for operating an add-onbattery system is shown. The method of FIG. 9 may be included in thesystem of FIGS. 1-4 as executable instructions stored in non-transitorymemory. Further, the method of FIGS. 9-15 may provide the operatingsequences shown in FIGS. 5-8.

At 901, method 900 determines operating characteristics of the externalelectrical energy storage device. In one example, the externalelectrical energy storage device's operating characteristics may beretrieved from controller memory. The external electrical energy storagedevice operating characteristics may include but are not limited tocharging voltage (e.g., voltage applied to a battery for charging), fullcharge voltage (e.g., voltage where SOC is 100), type of battery (e.g.,lead/acid, Ni/Cd), and charging and discharging current limits. A usermay input the operating characteristics of the external electricalenergy storage device via a keyboard or user interface. Alternatively,the external electrical energy storage device operating characteristicsmay be programmed into memory at the time of manufacture. Method 900proceeds to 902 after external electrical energy storage deviceoperating characteristics are determined.

At 902, method 900 judges if the add-on battery system is electricallycoupled to an external electrical energy storage device (e.g., a batteryexternal from the add-on battery system). In one example, method 900 mayjudge the add-on battery system is electrically coupled to an externalbattery via sensing presence or absence of voltage at pins of anelectrical connector configured to electrically couple the add-onbattery system to the external electrical energy storage device. Ifmethod 900 judges that the add-on battery system is electrically coupledto an external electrical energy storage device, the answer is yes andmethod 900 proceeds to 903. Otherwise, the answer is no and method 900proceeds to exit.

At 903, method 900 judges if the system the add-on battery system iselectrically coupled to a system that includes a load current sensor.The load current sensor may sense current that may flow between theexternal electrical energy storage device and a load powered by theexternal electrical energy storage device. If method 900 judges that theadd-on battery system is electrically coupled to a system that includesa load current sensor, the answer is yes and method 900 proceeds to 904of FIG. 10. Otherwise, the answer is no and method 900 proceeds to 920.

At 904, method 900 judges if there is current flow from the externalelectrical energy storage device to the electrical load powered by theexternal electrical energy storage device. In one example, method 900judges that there is current flow based on signed output from thecurrent sensor. If method 900 judges that there is current flow from theexternal electrical energy storage device to the electrical load poweredby the external electrical energy storage device, the answer is yes andmethod 900 proceeds to 905. Otherwise, the answer is no and method 900proceeds to 910.

At 905, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalbattery voltage, the answer is yes and method 900 proceeds to 906.Otherwise, the answer is no and method 900 proceeds to 907.

At 906 n method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe add-on battery system battery is stepped down so that the externalbattery may be charged using a desired voltage that is less than theadd-on battery system battery voltage. The desired voltage may beretrieved from memory and H-bridge MOSFET switching times may beadjusted to provide the desired voltage from the add-on battery systembattery to the external battery. Table 1 identifies transistor operatingstates for the transistors of FIG. 4 operating in buck mode transferringcharge from the add-on battery system battery to the external battery.

TABLE 1 Voltage H-bridge Q1 Q2 Q3 Q4 Current direction levelconfiguration Mode Mode Mode Mode Add-on to external Va < Ve BOOST ONOFF !PWM PWM Add-on to external Va > Ve BUCK PWM !PWM ON OFF External toadd-on Va < Ve BOOST !PWM PWM ON OFF External to add-on Va > Ve BUCK ONOFF PWM !PWMThe first column on the left side of table 1 indicates the direction ofcurrent flow. Current and electrical power flows from the add-on batterysystem battery to the external battery when current flow is indicated as“add-on to external.” Current and electrical power flows from theexternal battery to the add-on battery system battery when current flowis indicated as “external to add-on.” The second column indicatesvoltage level between the add-on battery system battery voltage (Va) andthe external system battery voltage (Ve) for the current direction. Thethird column indicates the H-bridge operating configuration thatsupplies current in the indicated direction according to the voltagelevels of the add-on battery system battery and the external systembattery. “BOOST” indicates the H-bridge is operated in a boost mode whentransistors Q1-Q4 of the system shown in FIG. 4 are operated accordingto the indicated transistor modes Q1-Q4 located in columns to the rightof the H-bridge configuration column. Transistors Q1-Q4 may not beoperated and indicated as “OFF,” operated in a continuous conductingstate “ON,” cycled conducting and not conducting via a pulse widthmodulated signal and indicated as “PWM,” or cycled conducting and notconducting via a near inverse pulse width modulation signal as indicatedas “!PWM.”

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in PWM mode, Q2 is operated in !PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the add-on battery system to the external electrical energy storagedevice to extend external electrical energy storage device operation.Further, since it has been determined at 904 that the external load isconsuming charge, a majority of add-on battery system charge supplied tothe external electrical energy storage device is directly transferred tothe electrical load that is electrically coupled to the externalelectrical energy storage device. Consequently, add-on battery systemcharge may be applied more efficiently as compared to simply chargingthe external electrical energy storage device. Method 900 proceeds toreturn to 920 of FIG. 9 after 906 completes.

At 907, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe add-on battery system battery is stepped up so that the externalbattery may be charged using a desired voltage that is greater than theadd-on battery system battery voltage. The desired voltage may beretrieved from memory and H-bridge MOSFET switching times may beadjusted to provide the desired voltage from the add-on battery systembattery to the external battery. Table 1 identifies transistor operatingstates for the transistors of FIG. 4 operating in boost modetransferring charge from the add-on battery system battery to theexternal battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated !PWM, and Q4 is operatedPWM. In this way, charge may be transferred from the add-on batterysystem to the external electrical energy storage device to extendexternal electrical energy storage device operation. Further, since ithas been determined at 904 that the external load is consuming charge, amajority of add-on battery system charge supplied to the externalelectrical energy storage device is directly transferred to theelectrical load that is electrically coupled to the external electricalenergy storage device. Consequently, add-on battery system charge may beapplied more efficiently as compared to simply charging the externalelectrical energy storage device. Method 900 proceeds to return to 920of FIG. 9 after 907 completes.

At 910, method 900 judges if current is flowing to the external energystorage device from a source external to the add-on battery system(e.g., a charger or a motor operating as a generator). In one example,method 900 judges that there is current flow based on signed output fromthe current sensor. If method 900 judges that there is current flow fromto the external electrical energy storage device, the answer is yes andmethod 900 proceeds to 911. Otherwise, the answer is no and method 900proceeds to 912.

At 912, method 900 ceases to operate the bi-directional DC/DC converterto transfer charge if the bi-directional DC/DC converter is transferringcharge based on current sensor output. If the bi-directional DC/DCconverter is transferring charge based on the voltage method of FIG. 15,the bi-directional DC/DC converter may continue to transfer charge.Thus, the bi-directional DC/DC converter may transfer or stoptransferring charge between the add-on battery system and the externalelectrical energy storage device in response to current sensor output.Method 900 proceeds to return to 920 of FIG. 9 after 912 completes.

At 911, method 900 judges if external energy storage device voltage isgreater than a third threshold. In one example, method 900 comparesexternal energy storage device voltage to the third threshold voltage.If method 900 judges that external energy storage device voltage isgreater than the third threshold voltage, the answer is yes and method900 proceeds to 913. Otherwise, the answer is no and method 900 proceedsto 912. In this way, method 900 may start add-on battery system batterycharging only after the external electrical energy storage device ischarged to a threshold level so that the external electrical energystorage device is given charging priority.

At 913, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalbattery voltage, the answer is yes and method 900 proceeds to 914.Otherwise, the answer is no and method 900 proceeds to 915.

At 914, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe external battery is stepped up so that the add-on battery systembattery may be charged using a desired voltage that is greater than theexternal battery voltage. The desired voltage may be retrieved frommemory and H-bridge MOSFET switching times may be adjusted to providethe desired voltage from the external battery to the add-on batterysystem battery. Table 1 identifies transistor operating states for thetransistors of FIG. 4 operating in boost mode transferring charge fromthe external battery to the add-on battery system battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in !PWM mode, Q2 is operated in PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the external electrical energy storage device to the add-on batterysystem battery to recharge the add-on battery system battery orelectrical energy storage device. Method 900 proceeds to return to 920of FIG. 9 after 914 completes.

At 915, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe external energy storage device is stepped down so that the add-onbattery system battery may be charged using a desired voltage that isless than the external energy storage device voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external battery. Table 1 identifies transistoroperating states for the transistors of FIG. 4 operating in buck modetransferring charge from the external energy storage device to theadd-on battery system battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in PWM mode, and Q4 isoperated in !PWM mode. In this way, charge may be transferred from theexternal electrical energy storage device to the add-on battery systembattery to recharge the add-on battery system battery or electricalenergy storage device. Method 900 proceeds to return to 920 of FIG. 9after 915 completes.

At 920, method 900 judges if a derivative method to operate thebi-directional DC/DC converter is active. The derivative method judgeswhether or not to operate the bi-directional DC/DC converter to transfercharge between the add-on battery system battery and the externalelectrical energy storage device voltage based on a rate of change inexternal electrical energy storage device voltage. In one example, a bitin memory may indicate whether or not the derivative method is active.If method 900 judges that the derivative method is active, the answer isyes and method 900 proceeds to 921 of FIG. 11. Otherwise, the answer isno and method 900 proceeds to 940.

At 921, method 900 determines a derivative of the external electricalenergy storage device voltage. The derivative may be determined bysampling the external electrical energy storage device voltage at afirst time and a second time and dividing the voltage difference by thetime between the two samples. The derivative may be expressed as:

$\frac{dV}{dt} = \frac{{V(k)} - {V\left( {k - 1} \right)}}{{t(k)} - {t\left( {k - 1} \right)}}$

where V represents the external electrical energy storage devicevoltage, k represents the sample number, and t represents time. Method900 proceeds to 922 after the derivative is determined.

At 922, method 900 judges whether or not there is a change in sign ofthe derivative determined at 921 from a previously determined derivativevalue during conditions where the derivative method has activated thebi-directional DC/DC converter. A change in sign may be indicative of achange in current direction from or to the external electrical energystorage device. Thus, a change in derivative sign may be a basis forceasing bi-directional DC/DC converter operation based on the derivativemethod because of absence or reduction of current flow into or out ofthe external electrical energy storage device. Further, in someexamples, method 900 may judge if the derivative value is greater than athreshold value in the presence of a change in the derivative sign. Thebi-directional DC/DC converter may deactivated in response to a changein the derivative sign and a substantial change in the derivative valuebecause these parameters may provide a strong indication of a change inload on the external electrical energy storage device. If method 900judges that there is a change in derivative sign after thebi-directional DC/DC converter has been activated by the derivativemethod, or alternatively, if there is a change in derivative sign afterthe bi-directional DC/DC converter has been activated by the derivativemethod and the absolute value of the derivative is greater than athreshold value, the answer is yes and method 900 proceeds to 928.Otherwise, the answer is no and method 900 proceeds to 923.

At 923, method 900 judges if the bi-directional DC/DC converter hasalready been activated and is presently active based on the derivativemethod beginning at 921. Method 900 judges which thresholds are to beapplied for the derivative method to determine if the derivative's valueis sufficient to activate the bi-directional DC/DC converter.

The change in external electrical energy storage device voltage may beindicative of current consumption via a load that is electricallycoupled to the external electrical energy storage device. For example,external electrical energy storage device voltage may be reduced inresponse to a load applied to the external electrical energy storagedevice. However, once the DC/DC converter is activated and suppliescharge to the external electrical energy storage device, the value ofthe derivative may be reduced. Likewise, external electrical energystorage device voltage may be increase in response to an external sourcecharging the external electrical energy storage device. However, oncethe bi-directional DC/DC converter is activated and transfers charge tothe add-on battery system battery, the value of the derivative may bereduced.

Method 900 may change a state of a bit in memory to indicate whether ornot the bi-directional DC/DC converter has been activated based on thederivative method. If method 900 judges that the bi-directional DC/DCconverter has been activated and that the bi-directional DC/DC converteris activated based on the derivative method, the answer is yes andmethod 900 proceeds to 926. Otherwise, the answer is no and method 900proceeds to 924.

At 924, method 900 judges whether or not the value of the derivativedetermined at 921 is less than a first threshold value. For example, ifthe derivative value is minus five and the threshold value is minus two,the answer is yes and method 900 proceeds to 929 of FIG. 12. Thederivative value being less than the first threshold value may beindicative of a substantial external load being applied to the externalelectrical energy storage device. If the answer is no, method 900proceeds to 925.

At 929, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalbattery voltage, the answer is yes and method 900 proceeds to 931.Otherwise, the answer is no and method 900 proceeds to 930.

At 931, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe add-on battery system battery is stepped down so that the externalelectrical energy storage device may be charged using a desired voltagethat is less than the add-on battery system battery voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external electrical energy storage device. Table 1identifies transistor operating states for the transistors of FIG. 4operating in buck mode transferring charge from the add-on batterysystem battery to the external battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in PWM mode, Q2 is operated in !PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the add-on battery system to the external electrical energy storagedevice to extend external electrical energy storage device operation.Further, since the derivative value indicates that the external load isconsuming charge, a majority of add-on battery system charge supplied tothe external electrical energy storage device is directly transferred tothe electrical load that is electrically coupled to the externalelectrical energy storage device. Consequently, add-on battery systemcharge may be applied more efficiently as compared to simply chargingthe external electrical energy storage device. Method 900 proceeds toreturn to 940 of FIG. 9 after 931 completes.

At 930, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe add-on battery system battery is stepped up so that the externalelectrical energy storage device may be charged using a desired voltagethat is greater than the add-on battery system battery voltage. Thedesired voltage may be retrieved from memory and H-bridge MOSFETswitching times may be adjusted to provide the desired voltage from theadd-on battery system battery to the external battery. Table 1identifies transistor operating states for the transistors of FIG. 4operating in boost mode transferring charge from the add-on batterysystem battery to the external battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in !PWM mode, and Q4 isoperated in PWM mode. In this way, charge may be transferred from theadd-on battery system to the external electrical energy storage deviceto extend external electrical energy storage device operation. Further,since the derivative value indicates that the external load is consumingcharge, a majority of add-on battery system charge supplied to theexternal electrical energy storage device is directly transferred to theelectrical load that is electrically coupled to the external electricalenergy storage device. Consequently, add-on battery system charge may beapplied more efficiently as compared to simply charging the externalelectrical energy storage device. Method 900 proceeds to return to 940of FIG. 9 after 930 completes.

At 925, method 900 judges whether or not the value of the derivativedetermined at 921 is greater than a second threshold value. For example,if the derivative value is five and the second threshold value is two,the answer is yes and method 900 proceeds to 933 of FIG. 12. Thederivative value being less than the first threshold value may beindicative of substantial external charging of the external electricalenergy storage device. If the answer is no, method 900 proceeds to 928.

At 933, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalenergy storage device voltage, the answer is yes and method 900 proceedsto 934. Otherwise, the answer is no and method 900 proceeds to 935.

At 934, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe external battery is stepped up so that the add-on battery systembattery may be charged using a desired voltage that is greater than theexternal battery voltage. The desired voltage may be retrieved frommemory and H-bridge MOSFET switching times may be adjusted to providethe desired voltage from the external battery to the add-on batterysystem battery. Table 1 identifies transistor operating states for thetransistors of FIG. 4 operating in boost mode transferring charge fromthe external battery to the add-on battery system battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in !PWM mode, Q2 is operated in PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the external electrical energy storage device to the add-on batterysystem battery to recharge the add-on battery system battery orelectrical energy storage device. Method 900 proceeds to return to 940of FIG. 9 after 934 completes.

At 935, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe external energy storage device is stepped down so that the add-onbattery system battery may be charged using a desired voltage that isless than the external energy storage device voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external battery. Table 1 identifies transistoroperating states for the transistors of FIG. 4 operating in buck modetransferring charge from the external energy storage device to theadd-on battery system battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in PWM mode, and Q4 isoperated in !PWM mode. In this way, charge may be transferred from theexternal electrical energy storage device to the add-on battery systembattery to recharge the add-on battery system battery or electricalenergy storage device. Method 900 proceeds to return to 940 of FIG. 9after 935 completes.

At 928, method 900 ceases to operate the bi-directional DC/DC converterto transfer charge between the add-on battery system and the externalelectrical energy storage device based on the derivative method. Method900 may continue to transfer charge between the add-on battery systemand the external electrical energy storage device based on the voltagemethod that begins at 960. Method 900 returns to 940 of FIG. 9 afterstep 928.

At 926, method 900 judges whether or not the value of the derivativedetermined at 921 is less than a third threshold value. The thirdthreshold value may be less than the first threshold at 924 because thebi-directional DC/DC converter may be supplying charge to the externalelectrical energy storage device. Consequently, the derivative value maybe reduced, yet it still may be desirable to operate the bi-directionalDC/DC converter. If the derivative value is greater than the thirdthreshold value, the answer is yes and method 900 proceeds to 929 ofFIG. 12. If the answer is no, method 900 proceeds to 927.

At 927, method 900 judges whether or not the value of the derivativedetermined at 921 is greater than a fourth threshold value. The fourththreshold value may be less than the second threshold value at 925because the bi-directional DC/DC converter may be supplying charge tothe add-on battery system battery. Consequently, the derivative valuemay be reduced, yet it still may be desirable to operate thebi-directional DC/DC converter. If the derivative value is less than thefourth threshold value, the answer is yes and method 900 proceeds to 933of FIG. 12. If the answer is no, method 900 proceeds to 928.

At 940, method 900 judges if a rolling average method to operate thebi-directional DC/DC converter is active. The rolling average methodjudges whether or not to operate the bi-directional DC/DC converter totransfer charge between the add-on battery system battery and theexternal electrical energy storage device voltage based on a differencein a rolling average of the external electrical energy storage devicevoltage and instantaneous external electrical energy device voltage. Inone example, a bit in memory may indicate whether or not the rollingaverage method is active. The rolling average method may be activatedvia a user interface or programming during manufacture. If method 900judges that the rolling average method is active, the answer is yes andmethod 900 proceeds to 941 of FIG. 13. Otherwise, the answer is no andmethod 900 proceeds to 960.

At 941, method 900 determines a rolling average of external energystorage device voltage. The rolling average may be based on apredetermined number of voltage samples (e.g., 5) observed at apredetermined frequency (e.g., 0.5 Hz). The rolling average may beexpressed as:

$V_{ave} = \frac{{V(k)} + {V\left( {k - 1} \right)} + {V\left( {k - 2} \right)} + {V\left( {k - 3} \right)} + {V\left( {k - 4} \right)}}{5}$

where V represents the external electrical energy storage devicevoltage, and k represents the sample number. This example is based onfive samples, but a different number of samples may be used if desired.Method 900 proceeds to 942 after the rolling average is determined.

At 942, method 900 determines instantaneous external energy storagedevice voltage. Instantaneous external energy storage device voltage isdetermined by sampling instantaneous external energy storage devicevoltage at the present time. Method 900 proceeds to 943 after theinstantaneous external energy storage device voltage is determined.

At 943, method 900 judges whether or not there is a change in sign of anerror value determined from the rolling average. The error is theinstantaneous external energy storage device voltage determined at 942minus the rolling average determined at 941. The sign value of the errormay be stored to memory each time the error is determined and it may becompared to the sign of the previously determined error value. A changein sign may be indicative of a change in current direction from or tothe external electrical energy storage device. Thus, a change in errorsign may be a basis for ceasing bi-directional DC/DC converter operationbased on the derivative method because of absence or reduction ofcurrent flow into or out of the external electrical energy storagedevice. Further, in some examples, method 900 may judge if the errorvalue is greater than a threshold value in the presence of a change inthe error sign. The bi-directional DC/DC converter may deactivated inresponse to a change in the error sign and a substantial change in theerror value because these parameters may provide a strong indication ofa change in load on the external electrical energy storage device. Ifmethod 900 judges that there is a change in error sign after thebi-directional DC/DC converter has been activated by the rolling averagemethod, or alternatively, if there is a change in error sign after thebi-directional DC/DC converter has been activated by the rolling averagemethod and the absolute value of the error is greater than a thresholdvalue, the answer is yes and method 900 proceeds to 949. Otherwise, theanswer is no and method 900 proceeds to 944.

At 944, method 900 judges if the bi-directional DC/DC converter hasalready been activated and is presently active based on the rollingaverage method beginning at 941. Method 900 judges which thresholds areto be applied for the rolling average method to determine if the rollingaverage value is sufficient to activate the bi-directional DC/DCconverter.

Method 900 may change a state of a bit in memory to indicate whether ornot the bi-directional DC/DC converter has been activated based on therolling average method. If method 900 judges that the bi-directionalDC/DC converter has been activated and that the bi-directional DC/DCconverter is activated based on the rolling average method, the answeris yes and method 900 proceeds to 947. Otherwise, the answer is no andmethod 900 proceeds to 945.

At 945, method 900 judges whether or not the value of the errordetermined at 943 (e.g., instantaneous voltage minus rolling averagevoltage) is less than a first threshold value. For example, if the erroris minus five and the threshold value is minus two, the answer is yesand method 900 proceeds to 950 of FIG. 14. The error value being lessthan the first threshold value may be indicative of a substantialexternal load being applied to the external electrical energy storagedevice. If the answer is no, method 900 proceeds to 946.

At 950, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalbattery voltage, the answer is yes and method 900 proceeds to 951.Otherwise, the answer is no and method 900 proceeds to 952.

At 951, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe add-on battery system battery is stepped down so that the externalelectrical energy storage device may be charged using a desired voltagethat is less than the add-on battery system battery voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external electrical energy storage device. Table 1identifies transistor operating states for the transistors of FIG. 4operating in buck mode transferring charge from the add-on batterysystem battery to the external battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in PWM mode, Q2 is operated in !PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the add-on battery system to the external electrical energy storagedevice to extend external electrical energy storage device operation.Further, since the derivative value indicates that the external load isconsuming charge, a majority of add-on battery system charge supplied tothe external electrical energy storage device is directly transferred tothe electrical load that is electrically coupled to the externalelectrical energy storage device. Consequently, add-on battery systemcharge may be applied more efficiently as compared to simply chargingthe external electrical energy storage device. Method 900 proceeds toreturn to 960 of FIG. 9 after 951 completes.

At 952, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe add-on battery system battery is stepped up so that the externalelectrical energy storage device may be charged using a desired voltagethat is greater than the add-on battery system battery voltage. Thedesired voltage may be retrieved from memory and H-bridge MOSFETswitching times may be adjusted to provide the desired voltage from theadd-on battery system battery to the external battery. Table 1identifies transistor operating states for the transistors of FIG. 4operating in boost mode transferring charge from the add-on batterysystem battery to the external battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in !PWM mode, and Q4 isoperated in PWM mode. In this way, charge may be transferred from theadd-on battery system to the external electrical energy storage deviceto extend external electrical energy storage device operation. Further,since the derivative value indicates that the external load is consumingcharge, a majority of add-on battery system charge supplied to theexternal electrical energy storage device is directly transferred to theelectrical load that is electrically coupled to the external electricalenergy storage device. Consequently, add-on battery system charge may beapplied more efficiently as compared to simply charging the externalelectrical energy storage device. Method 900 proceeds to return to 960of FIG. 9 after 952 completes.

At 946, method 900 judges whether or not the value of the errordetermined at 943 is greater than a second threshold value. For example,if the error value is five and the second threshold value is two, theanswer is yes and method 900 proceeds to 954 of FIG. 14. The error valuebeing less than the first threshold value may be indicative ofsubstantial external charging of the external electrical energy storagedevice. If the answer is no, method 900 proceeds to 949.

At 954, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalenergy storage device voltage, the answer is yes and method 900 proceedsto 955. Otherwise, the answer is no and method 900 proceeds to 956.

At 955, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe external battery is stepped up so that the add-on battery systembattery may be charged using a desired voltage that is greater than theexternal battery voltage. The desired voltage may be retrieved frommemory and H-bridge MOSFET switching times may be adjusted to providethe desired voltage from the external battery to the add-on batterysystem battery. Table 1 identifies transistor operating states for thetransistors of FIG. 4 operating in boost mode transferring charge fromthe external battery to the add-on battery system battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in !PWM mode, Q2 is operated in PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the external electrical energy storage device to the add-on batterysystem battery to recharge the add-on battery system battery orelectrical energy storage device. Method 900 proceeds to return to 960of FIG. 9 after 955 completes.

At 956, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe external energy storage device is stepped down so that the add-onbattery system battery may be charged using a desired voltage that isless than the external energy storage device voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external battery. Table 1 identifies transistoroperating states for the transistors of FIG. 4 operating in buck modetransferring charge from the external energy storage device to theadd-on battery system battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in PWM mode, and Q4 isoperated in !PWM mode. In this way, charge may be transferred from theexternal electrical energy storage device to the add-on battery systembattery to recharge the add-on battery system battery or electricalenergy storage device. Method 900 proceeds to return to 960 of FIG. 9after 956 completes.

At 949, method 900 ceases to operate the bi-directional DC/DC converterto transfer charge between the add-on battery system and the externalelectrical energy storage device based on the rolling average method.Method 900 may continue to transfer charge between the add-on batterysystem and the external electrical energy storage device based on thevoltage method that begins at 960. Method 900 returns to 960 of FIG. 9after step 949.

At 947, method 900 judges whether or not the value of the errordetermined at 943 is less than a third threshold value. The thirdthreshold value may be less than the first threshold at 945 because thebi-directional DC/DC converter may be supplying charge to the externalelectrical energy storage device. Consequently, the error value may bereduced, yet it still may be desirable to operate the bi-directionalDC/DC converter. If the error value is greater than the third thresholdvalue, the answer is yes and method 900 proceeds to 950 of FIG. 14. Ifthe answer is no, method 900 proceeds to 948.

At 948, method 900 judges whether or not the value of the errordetermined at 943 is greater than a fourth threshold value. The fourththreshold value may be less than the second threshold value at 946because the bi-directional DC/DC converter may be supplying charge tothe add-on battery system battery. Consequently, the error value may bereduced, yet it still may be desirable to operate the bi-directionalDC/DC converter. If the error value is less than the fourth thresholdvalue, the answer is yes and method 900 proceeds to 954 of FIG. 14. Ifthe answer is no, method 900 proceeds to 949.

At 960, method 900 determines a desired external energy storage devicevoltage. In one example, method 900 selects a desired external energystorage device voltage based on a desired state of charge (SOC) for theexternal energy storage device. For example, if output voltage of abattery is 25.25 volts at 70% SOC, method 900 selects 25.25 volts as thedesired external energy storage device voltage. The desired externalenergy storage device voltage may be based on battery chemistry (e.g.,lead-acid), number of battery cells, battery temperature, and otherconditions. The desired external energy storage device voltage may bestored in memory during manufacturing or it may be input via a userinterface. Method 900 proceeds to 961 after the desired external energystorage device voltage is determined.

At 961, method 900 judges if the external electrical energy storagedevice voltage is greater than a first threshold voltage. In oneexample, the first threshold voltage is an open circuit voltage of theexternal electrical energy storage device when the external electricalenergy storage device is at 100% SOC, but below a float voltage limit ofa battery charger. A voltage of the external electrical energy storagedevice being greater than the first threshold voltage may be indicativeof a condition desirable for charging the add-on battery system battery.If method 900 judges that the external electrical energy storage devicevoltage is greater than a first threshold voltage, the answer is yes andmethod 900 proceeds to 965 of FIG. 15. Otherwise, the answer is no andmethod 900 proceeds to 962.

At 965, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalenergy storage device voltage, the answer is yes and method 900 proceedsto 966. Otherwise, the answer is no and method 900 proceeds to 967.

At 966, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe external battery is stepped up so that the add-on battery systembattery may be charged using a desired voltage that is greater than theexternal battery voltage. The desired voltage may be retrieved frommemory and H-bridge MOSFET switching times may be adjusted to providethe desired voltage from the external battery to the add-on batterysystem battery. Table 1 identifies transistor operating states for thetransistors of FIG. 4 operating in boost mode transferring charge fromthe external battery to the add-on battery system battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in !PWM mode, Q2 is operated in PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the external electrical energy storage device to the add-on batterysystem battery to recharge the add-on battery system battery orelectrical energy storage device. Method 900 exits at FIG. 9 after step966 completes.

At 967, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe external energy storage device is stepped down so that the add-onbattery system battery may be charged using a desired voltage that isless than the external energy storage device voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external battery. Table 1 identifies transistoroperating states for the transistors of FIG. 4 operating in buck modetransferring charge from the external energy storage device to theadd-on battery system battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in PWM mode, and Q4 isoperated in !PWM mode. In this way, charge may be transferred from theexternal electrical energy storage device to the add-on battery systembattery to recharge the add-on battery system battery or electricalenergy storage device. Method 900 exits at FIG. 9 after step 966completes.

At 962, method 900 judges if the external electrical energy storagedevice voltage is less than a second threshold voltage. In one example,the second threshold voltage is an open circuit voltage of the externalelectrical energy storage device when the external electrical energystorage device is a predetermined SOC that provides sufficient chargecapacity in the external electrical energy storage device to operate theexternal load. For example, the second threshold voltage may be avoltage that corresponds to 40% SOC for the external electrical energystorage device. If method 900 judges that the external electrical energystorage device voltage is less than a second threshold voltage, theanswer is yes and method 900 proceeds to 965 of FIG. 15. Otherwise, theanswer is no and method 900 proceeds to 963.

At 970, method 900 judges if the add-on battery system battery voltageis greater than the external electrical energy storage device voltage(e.g., battery voltage). In one example, method 900 samples add-onbattery system battery voltage and external battery voltage. If theadd-on battery system battery voltage is greater than the externalbattery voltage, the answer is yes and method 900 proceeds to 971.Otherwise, the answer is no and method 900 proceeds to 972.

At 971, method 900 operates the add-on battery system bi-directionalDC/DC converter in a buck mode. In buck mode, the higher voltage fromthe add-on battery system battery is stepped down so that the externalelectrical energy storage device may be charged using a desired voltagethat is less than the add-on battery system battery voltage. The desiredvoltage may be retrieved from memory and H-bridge MOSFET switching timesmay be adjusted to provide the desired voltage from the add-on batterysystem battery to the external electrical energy storage device. Table 1identifies transistor operating states for the transistors of FIG. 4operating in buck mode transferring charge from the add-on batterysystem battery to the external battery.

Thus, to operate the H-bridge in a buck mode when add-on battery systembattery voltage is greater than external electrical energy storagedevice voltage, Q1 is operated in PWM mode, Q2 is operated in !PWM mode,Q3 is operated ON, and Q4 is OFF. In this way, charge may be transferredfrom the add-on battery system to the external electrical energy storagedevice to extend external electrical energy storage device operation.Further, since the derivative value indicates that the external load isconsuming charge, a majority of add-on battery system charge supplied tothe external electrical energy storage device is directly transferred tothe electrical load that is electrically coupled to the externalelectrical energy storage device. Consequently, add-on battery systemcharge may be applied more efficiently as compared to simply chargingthe external electrical energy storage device. Method 900 proceeds toexit after 971 completes.

At 972, method 900 operates the add-on battery system bi-directionalDC/DC converter in a boost mode. In boost mode, the lower voltage fromthe add-on battery system battery is stepped up so that the externalelectrical energy storage device may be charged using a desired voltagethat is greater than the add-on battery system battery voltage. Thedesired voltage may be retrieved from memory and H-bridge MOSFETswitching times may be adjusted to provide the desired voltage from theadd-on battery system battery to the external battery. Table 1identifies transistor operating states for the transistors of FIG. 4operating in boost mode transferring charge from the add-on batterysystem battery to the external battery.

Thus, to operate the H-bridge in a boost mode when add-on battery systembattery voltage is less than external electrical energy storage devicevoltage, Q1 is ON, Q2 is OFF, Q3 is operated in !PWM mode, and Q4 isoperated in PWM mode. In this way, charge may be transferred from theadd-on battery system to the external electrical energy storage deviceto extend external electrical energy storage device operation. Further,since the derivative value indicates that the external load is consumingcharge, a majority of add-on battery system charge supplied to theexternal electrical energy storage device is directly transferred to theelectrical load that is electrically coupled to the external electricalenergy storage device. Consequently, add-on battery system charge may beapplied more efficiently as compared to simply charging the externalelectrical energy storage device. Method 900 proceeds to exit after 972completes.

At 963, method 900 deactivates the bi-directional DC/DC converter toconserve power in the add-on battery system. The bi-directional DC/DCconverter may be deactivated when the external load is not makingdemands on the external electrical energy storage device. Further thebi-directional DC/DC converter may be deactivated when the externalelectrical energy storage device is within a desired range of charge.

It should also be noted that the PWM frequency and duty cycle may beadjusted to provide different voltage or current gains from thebi-directional DC/DC converter. For example, the PWM duty cycle may beadjusted to increase bi-directional DC/DC converter voltage gainproportionately to a difference between external electrical energystorage device voltage and a threshold voltage. Further, the add-onbattery system output power may be limited to less than a thresholdamount of power so that size of conductors electrically coupling theadd-on battery system to the external electrical energy storage devicemay be desired gauge. Additionally, the bi-directional DC/DC voltageoutput during the various described modes may be limited to a thresholdvoltage greater than the present external electrical energy storagedevice voltage so that fuel meter device operation in the externalsystem may not be interfered by the add-on battery system.

Thus, the method of FIGS. 9-15 provides for an add-on battery operatingmethod, comprising: supplying electrical power from a first electricalenergy storage device in a first system to a second electrical energystorage device of a second system in response to an indication of anelectric load consuming charge from the second electrical energy storagedevice, the second system not part of the first system, the electricalpower supplied via a bi-directional DC/DC converter in the first system.The method includes where the indication of the electrical loadconsuming charge is based on a derivative of a voltage of the secondelectrical energy storage device. The method includes where theindication of the electrical load consuming charge is based on adifference of a rolling average of voltage of the second electricalenergy storage device and an instantaneous voltage of the secondelectrical energy storage device.

In some examples, the method further comprises supplying the electricalpower from the first electrical energy storage device to the secondelectrical energy storage device in response to a voltage of the secondelectrical energy storage device. The method includes where theelectrical power from the first electrical energy storage device issupplied to the second electrical energy storage device in response tothe voltage of the second electrical energy storage device being lessthan a threshold voltage. The method includes where the indication ofthe electric load consuming charge from the second electrical energystorage device is based on a current sensor in the second system. Themethod includes where the first system is portable, and where the firstsystem includes a battery having a higher charge density than a batteryof the second system.

The method of FIGS. 9-15 also provides for an add-on battery operatingmethod, comprising: in a first mode, supplying electrical power from afirst electrical energy storage device in a first system to a secondelectrical energy storage device of a second system in response to anindication of an electric load consuming charge from the secondelectrical energy storage device, the second system not part of thefirst system, the electrical power supplied via a bi-directional DC/DCconverter in the first system; and in a second mode, supplyingelectrical power from the second electrical energy storage device of thesecond system to the first electrical energy storage device in the firstsystem via the bi-directional DC/DC converter in response to anindication the second electrical energy storage device is charged to alevel greater than a threshold level.

In some examples, the method includes where the indication of electricload consuming charge from the second electrical energy storage deviceis based on a voltage of the second electrical energy storage device.The method includes where the indication of electric load consumingcharge from the second electrical energy storage device is further basedon a derivative of the voltage of the second electrical energy storagedevice. The method includes where the indication of electric loadconsuming charge from the second electrical energy storage device isfurther based on a rolling average of the voltage of the secondelectrical energy storage device. The method further comprises supplyingelectrical power from the second electrical energy storage device of thesecond system to the first electrical energy storage device in the firstsystem in response to a derivative of a voltage of the second electricalenergy storage device.

Note that the example control and estimation routines included hereincan be applied to various add-on battery system configurations. Thecontrol methods and routines disclosed herein may be stored asexecutable instructions in non-transitory memory and may be carried outby the control system including the controller in combination with thevarious sensors, actuators, and other hardware. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations,and/or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the features andadvantages of the example embodiments described herein, but is providedfor ease of illustration and description. One or more of the illustratedactions, operations and/or functions may be repeatedly performeddepending on the particular strategy being used. Further, the describedactions, operations and/or functions may graphically represent code tobe programmed into non-transitory memory of the computer readablestorage medium in the control system, where the described actions arecarried out by executing the instructions in a system including thevarious hardware components in combination with the electroniccontroller

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,the add-on battery system may be applied to battery powered systems thatmay rely on a variety of different battery chemistries.

1. An add-on battery system, comprising: a battery; a bi-directional DC/DC converter including an H-bridge comprising a transformer positioned in a horizontal circuit extending between two vertical circuits, and a controller including instructions stored in non-transitory memory to direct current flow into the battery and out of the battery via the bi-directional DC/DC converter in response to conditions of an external electrical energy storage device.
 2. The add-on battery system of claim 1, further comprising: additional instructions to activate the bi-directional DC/DC converter in response to a derivative of a voltage of the external electrical energy storage device.
 3. The add-on battery system of claim 1, further comprising: additional instructions to activate the bi-directional DC/DC converter in response to an error between a rolling average of a voltage of the external electrical energy storage device and an instantaneous voltage of the external electrical energy storage device.
 4. The add-on battery system of claim 1, further comprising: two operational amplifiers and an inductor, the two operational amplifiers and the inductor directly coupled to the transformer.
 5. The add-on battery system of claim 4, where the inductor is positioned in the horizontal circuit between the transformer and the two additional transistors in the second vertical circuits.
 6. The add-on battery system of claim 5, where the transformer includes two windings, where a first of the two windings is coupled to the two operational amplifiers, and where a second of the two windings is coupled to the inductor and the two transistors in the first vertical circuit.
 7. The add-on battery system of claim 1, further comprising: additional instructions to deactivate the bi-directional DC/DC converter in response to a voltage of the external electrical energy storage device being less than a first threshold and being greater than a second threshold.
 8. The add-on battery system of claim 7, further comprising: additional instructions to; activate the bi-directional DC/DC converter in response to the voltage of the external electrical energy storage device being greater than the first threshold and being less than the second threshold.
 9. An add-on battery system, comprising: a battery; a bi-directional DC/DC converter including an H-bridge comprising a transformer positioned in a horizontal circuit extending between two vertical circuits; a controller including instructions stored in non-transitory memory to direct current flow into the battery and out of the battery via the bi-directional DC/DC converter in response to conditions of an external electrical energy storage device; and additional instructions to activate the bi-directional DC/DC converter in response to the voltage of the external electrical energy storage device being greater than the first threshold and being less than the second threshold.
 10. The add-on battery system of claim 9, further comprising: additional instructions to activate the bi-directional DC/DC converter in response to a derivative of a voltage of the external electrical energy storage device.
 11. The add-on battery system of claim 9, further comprising: additional instructions to activate the bi-directional DC/DC converter in response to an error between a rolling average of a voltage of the external electrical energy storage device and an instantaneous voltage of the external electrical energy storage device.
 12. The add-on battery system of claim 9, where the bi-directional DC/DC converter includes an H-bridge comprising a transformer positioned in a horizontal circuit extending between two vertical circuits, a first of the two vertical circuit including two transistors and a second of the two vertical circuits including two additional transitors.
 13. The add-on battery system of claim 12, where the two vertical circuits are comprised of metal oxide semiconductor field effect transistors.
 14. The add-on battery system of claim 13, where the horizontal circuit further comprises an inductor.
 15. The add-on battery system of claim 9, further comprising: additional instructions to deactivate the bi-directional DC/DC converter in response to a voltage of the external electrical energy storage device being less than a first threshold and being greater than a second threshold.
 16. An add-on battery operating method, comprising: supplying electrical power from a first electrical energy storage device in a first system to a second electrical energy storage device in a second system in response to electric current being supplied from the second electrical energy storage device to an external load, the second system not part of the first system, the electrical power supplied via a bi-directional DC/DC converter in the first system; and supplying the electrical power from the first electrical energy storage device to the second electrical energy storage device in response to a voltage of the second electrical energy storage device being less than a threshold voltage.
 17. The method of claim 16, further comprising: operating the bi-directional DC/DC converter in a buck mode in response to a voltage of the first electrical energy storage device being greater than a voltage of the second electrical energy storage device, where a first, a second, and a third of four transistors of an H-bridge in the bi-directional DC/DC converter operate in the buck mode and where a fourth of the four transistors in the H-bridge does not operate in the buck mode.
 18. The method of claim 17, further comprising: operating the bi-directional DC/DC converter in a boost mode in response to the voltage of the first electrical energy storage device being less than the voltage of the second electrical energy storage device, where the second of the four transistors of the H-bridge in the bi-directional DC/DC converter does not operate in the boost mode and where the first, the third, and the fourth of the four transistors in the H-bridge operate in the boost mode. 